1. Field of the Invention
The present invention relates generally to an electrophotographic image forming apparatus for use in a copying machine, printer, facsimile or the like and, more particularly, to an electrophotographic image forming apparatus in which power is supplied from a main unit to a heater for heating a photosensitive drum so as to be maintained at a specified temperature, provided in the photosensitive drum in a drum unit, through a connector disposed between the main unit and the drum unit.
2. Description of the Prior Art
In such an electrophotographic image forming apparatus, it is known to perform the following basic electrophotographic process in which reverse development is employed:
Firstly, the surface of a photosensitive drum is uniformly charged by a charger in a dark place. For exposure, light is then directed from an exposure device to an image portion formed on the surface of the photosensitive drum thus uniformly charged and the electrical charge on the illuminated image portion is eliminated to decrease the electric potential, thereby forming an electrostatic latent image having square well potential. Thereafter, toner, which is charged to the same polarity as that of the uniform charge applied to the surface of the photosensitive drum, is adhered to the electrostatic latent image by a developing device, utilizing an electric field formed by a developing bias, in order to form a visible toner image. A transfer sheet is subsequently overlaid on the toner image and electrical charge opposite in polarity to the charge of the toner is applied to the transfer sheet by a transferring device so that the toner image is transferred onto the transfer sheet owing to static electricity. The transfer sheet is further discharged by a discharging separator to reduce the electrostatic attraction force effecting on the photosensitive drum so that the transfer sheet is separated from the photosensitive drum. Thereafter, heat and/or pressure are applied by the fixing device to the toner image which has been transferred onto the transfer sheet so as to be fused to form a permanent image.
The surface of the photosensitive drum is provided with a photosensitive layer formed by vapor-depositing or coating a photoconductive material. As the photoconductive material, metals such as Se, CdS have been often used, but these metals are injurious to the human body and therefore cost a great deal for ensuring safety during the manufacturing process and waste disposal. In order to overcome the disadvantage, an organic photoconductor, amorphous silicon is widely used instead of these metals in recent years. The photosensitive drum having a photosensitive layer formed from amorphous silicon has been employed for use in electrophotographic image apparatus which are continuously used for a long period of time, because it has excellent surface hardness and a long service life. However, the charge intensity of such an amorphous silicon photosensitive layer when charged at room temperature is weak, so that the potential difference between an illuminated portion and a non-illuminated portion is not sufficient, which leads to the degradation of image quality.
One approach to solving the above problem inherent to a photosensitive drum having an amorphous silicon photosensitive layer has been provision of a heater within the photosensitive drum. This heater is supplied with power from a main unit through a connecter disposed between the main unit and a drum unit laving the photosensitive drum. By the use of the heater, the photosensitive drum is heated to and maintained at a specified temperature, and the charge intensity is thus improved.